At the end of the 20th century, scientists have revealed the biological causes of aging, and why it is so widespread among animals. It has also become apparent why different mammalian species have very different longevities. Aging is accompanied by changes in a wide range of cells, tissues, and organs. These include damage in DNA, proteins, membranes, and organelles, as well as the accumulation of high molecular weight insoluble aggregates. The multiple phenotypic changes that accompany aging show that there must also be many different causes. The failure to maintain a steady-state level of damage is the result of a limit to the resources that can be used to preserve the integrity of the soma. For each species, there is a tradeoff between what is invested in reproduction and what is used to maintain its cells, tissues, and organs. The failure of maintenance is irreversible, although longevity may be modulated under certain circumstances, such as dietary restriction accompanied by a loss of fertility.